both are similar in signature.
o2 you can place an odac inside (where battery would go) or use it on battery. magni is plug in from wall and modi sits on top if you use it.
for extremely sensitive cans you get more fine adjust on volume on the o2 but you should be able to also lower windows volume to a lower level and get more on the magni if using easier to power headphones. the magni has quite a bit more power output than the o2 which is why it always gets suggested for "budget" solutions with people wanting something a bit harder to drive than average.
while i wouldnt say the dt990 is hard to drive, its not as easy as some other headphones which is why you're having a little bit of an issue.
also make sure you know the difference...